High-density recording is feasible by using laser beam for read/write operation since such an operation is achieved by, in principle, focusing light on a spot having a diameter substantially equal to the wavelength of the laser beam used. Typically used in the optical recording are an optical disk (hereinafter referred to as "OD") as an information recording medium and an optical pickup using a laser device as means for achieving writing to and reading out of the OD.
Pits provided in the OD and storing information signals are of about 0.9 .mu.m in size each, and lines of pits, or tracks, are arranged with a narrow pitch of about 1.6 .mu.m. Accordingly, in a practical pickup operation, the detection of pits must be achieved while correcting a displacement of a track in a direction perpendicular to the tracks or a focusing deviation due to irregularities caused by revolution of the OD.
A conventional pickup employs an optical element, such as half mirror or hologram, as a beam splitter and is adapted to split light reflected from an OD to detect pits. In this case, a displacement of a track in a direction perpendicular to the tracks in the plane of the OD is corrected by, for example, the 3-beam method in which light from one laser diode (hereinafter referred to as "LD") as a light source is split into three beams by diffraction grating and these three beams are used to correct errors, while a focusing error is detected by, for example, the astigmatism method using a cylindrical lens.
Alternatively, there is developed a SCOOP (Self Coupled Optical Pickup ) method as disclosed, for example, Japanese Unexamined Patent Publication No. 72688/1991, wherein light reflected by an OD is returned to an LD (such light will be referred to as "returning light") and a signal is detected utilizing a change in oscillation state of the LD due to returning light. The objective of the SCOOP method is to reduce the number of optical components, such as beam splitter and cylindrical lens, as used in the aforesaid conventional pickup, to reduce the cost, and to facilitate precise positioning.
In this SCOOP method, there are used, for example, five LDs 41 to 45 which are linearly arranged as shown in FIG. 6. The central LD 43 is for detecting recorded signals, LDs 42 and 44 on both sides of LD 43 are each for detecting tracking error signals, and the outermost LDs 41 and 45 are each for detecting focusing error signals and are slightly shifted in opposite directions along the optical axis. These LDs for detecting error signals allow tracking errors and focusing errors to be detected.
Although the SCOOP method is able to detect signals with a simplified, less costly device without using expensive optical components, the oscillation of an LD is disturbed by returning light to make the LD instable. In addition, in detecting error signals or RF signals according to the SCOOP method, when the oscillation of the LD is disturbed to generate noise, a signal component is negated by such noise since a very fine fluctuation must be detected as the signal component. This results in a problem of inaccurate detection.